Pages

Thursday, January 10, 2013

PV-Thermal evaluation by Solar Blogger

9 Jan 2013: There's an interesting article on the Solar Blogger website by Stuart Elmes. It's about PVT (Photo-Voltaic-Thermal) panels, and he has a more technical PDF linked to it. The link was sent to me by David B. of Newform.

PVT panels are, in essence, a PV panel with a mesh of liquid cooling tubes behind the PV surface. For higher PV capture, the panel is uncovered, but for a higher thermal capture, the panel has a glass cover over it and insulation behind.

I am a great believer in PVT, but the only properly productive application for it that I can see is when it is associated with low-grade thermal storage and heat pump.
It could also good for those who have a limited area of roof, and haven't enough space. If you have enough roof surface to play with, the best place for PV might not be the best place for thermal. Ideally, a house needs 28-30 square metres of PV to attain the 4 kW that is recommended for the Feed-in-Tariff in the UK - a smooth, unobstructed and unshaded area facing south is the best. PV arrays normally have an air space under them and they work best on sunny days with a cooling northerly breeze that can get under the panels.
For solar thermal, the size can vary from 1sqm to 4sqm. Above that, it gets a bit silly unless you are addicted to hot showers all summer. So Solar thermal can often work well in smaller locations on hipped roofs, or large dormers, providing you can get a south aspect. Another criterion for best place might be closer to the optimum positions of a tank and the plumbing.
The idea of PVT is that you get a larger area for thermal, and the PV element is cooled by the liquid passing under it.

The paradox is this: for you to get useful heat to a water tank, you want the panel to run very hot, but for the PV panel to run well you want it to run very cool. Stuart's article explains the paradox well.

So for a solar thermal water tank you want the circuit to run at 50-70ºC, and when the tank reached 60ºC, the circulation would stop. If you ran PV at these temperatures it would be unworkable. If you want PV to be optimised, the purpose of the liquid has to be to cool the PV, so it should be circulating at 15-20ºC. A normally air cooled PV panel reaches 40-50ºC surface temperatures on a hot summer's day, but the PV cells run better is the wind is blowing.
If the water tank reaches optimum temperature and turns off the pump, the sun will make the PV even hotter, and as there would be insulation behind the PVT panel, there's nothing to stop the liquid temperature getting dangerously high. Stuart's article refers to the risk of temperatures as high as 220ºC. (I am informed by Newform that this might be a mistake, as Powervolt panels permitted to stagnate seriously haven't gone higher than 100ºC, although the glass topped Powertherms might get hotter.)

Therefore, for thermal storage for a heat pump, the panels running at 15-20ºC would be warming an infinitely large tank size, the earth, whose base temperature would be 10-12ºC, so there will always be a benefit. There is no risk of any of the system reaching stasis and turning off because earth has a vast thermal capacity in relation to the size of the panel array or the size of the building above it.

Having a larger surface area than normal solar thermal panels (28sqm instead of just 2 or 4sqm) the solar thermal gain for the store - large volume of low grade thermal energy is going to be very good! One thing that my experiment has shown is that the ground does not get 'hotter', but the volume of earth that is warmed up to a moderately warm temperature gets larger.

One thing about any of these systems, is that they must keep running. If the system circulates reliably everything is the right temperature. If a future owner of this house just turned the solar circulation off in summer, there would be problems if temperatures rise and there isn't sufficient capacity for expansion of the liquid.

In this house, we were within a gnat's whisker of having PVT, and I had already emailed Newform with preliminary arrangements for delivery and invoicing.

Four reasons we changed direction:

My wife rebelled against the cost of the PVT, when we have no real need for the extra electricity,

I had problems with the battery I intended to use for the PV power, and

I was offered a large amount of polycarbonate, aluminium and two large radiators, an offer so good, I had to accept it. I also had a good collection of OSB and timber, enough to make an entire sunbox without buying any materials other than plumbing connections.

I considered my research priorities, and the general thrust of these are to prove the efficacy of a black collector in a polycarbonate sunbox. With only 4 sqm of roof to choose from, there is only space for ONE more device - had to be a Sunbox.

March 2015: Work on the solar dehydrator has been progressing well, and it is nearly complete, lacking merely the top surface of the sola...

Peveril Solar House

Welcome to Charging the Earth!

PEVERIL SOLAR is the first house in the UK to be entirely solar heated all year round! It is Carbon Net-Zero. It is an 'Active House' balancing inputs and outputs. PV generation and heating system consumption are in favourable balance using concepts of energy storage. Others claiming houses to be the first date from 2013 (and are unbuilt); this house exists and was carbon zero since 2011.

The name 'Charging' refers to 'storing energy underground': we have custom-built solar collectors, Surya Sunboxes, with ETFE front surfaces, to pump solar heat deep down into the earth. The building's heat pump gets all of this back in Realtime (immediately), Diurnially (later during the evenings) or Interseasonally (in Winter, months after the Summer).

Thus, we are augmenting the heatpump by storing long term heat in the summer, and we are defrosting the ground in winter-spring conditions, supplying solar energy directly to the heat pump, through its ground loop.

The five-way pentangle of Grid, Borehole, Heat pump, PV roof and Sunboxes have made the house Carbon Zero (for metered consumption). It's working, and we will continue to record data, to maintain that efficiency, and write it up in this website through to next year and beyond.

During theAutumn of 2012, we built a small house extension that is ultra insulated, with a higher energy gain than it loses.

Note, that we still have a net import of power from the Grid, because we still need power for lighting, cooking and appliances. But for the building emissions (as opposed to lifestyle emissions), we have achieved a credit balance of the regulated quantities, as recorded by meters.

The web-log follows the project from this general idea in Aug. 2009 to a technology of Surya Sunboxes, which seem to be effective - reducing energy costs of the house. Some of the Tabs will help you to get background and theory. You can click below to 'Follow Blog' to get email notifications - or email me. Please add Comments to the blog entries. If you find items in the Glossary that need explaining better, please ask. Thankyou!

Publications

Equipment sponsor

Kingspan, for Varisol Tubes

Equipment Sponsor

MG Renewables

Equipment sponsor

Ice Energy Heat Pumps

Equipment Sponsor

Holscot, for ETFE panels, re-fronting the Sunbox

About the Author...

David Nicholson-Cole is a Lecturer in Architecture at the University of Nottingham, with 35 years experience of architectural teaching and practice, which has included special interests in construction, building information modelling, tall building design and renewable energy technologies.

Finally, thanks to my deceased aunt, Margaret Cringle (1915-2008) whose legacy paid for most of the cost of this project - as one who was always turning lights out to save electricity, she would be very pleased!